The present invention relates to an image forming apparatus for forming an image on a transfer material by an electrophotographic method and more particularly to an image forming apparatus for developing an image using a two-component developer.
In an example of an image forming process of forming an image by the electrophotographic method, an electrostatic latent image is formed on a photoreceptor, and the formed electrostatic latent image is developed by a developing device to form a toner image on the photoreceptor, and the formed toner image is transferred to a transfer material by a transfer device, and the transferred toner image is fixed on the transfer material by a fixing device, thus an image is formed on the transfer material. Further, in another example, the toner image on the photoreceptor is transferred to an intermediate transfer member, transferred to the transfer material from the intermediate transfer member by the transfer device, and is fixed, thus an image is formed on the transfer material.
At a developing step of the image forming process aforementioned, by using a two-component developer containing non-magnetic toner and a magnetic carrier and applying a developing bias voltage obtained by superimposing an AC bias voltage onto a DC bias voltage, the electrostatic latent image is developed.
In the developing device using the two-component developer, the toner and carrier are stirred in a developer container to frictionally charge the toner, and the toner frictionally charged is electrostatically adhered to the outer periphery of the carrier, is carried by a rotary developer carrying member having a built-in magnet, and is conveyed into a developing region, where the electrostatic latent image on the photoreceptor is developed by the charged toner.
To give the toner a charging characteristic, the surface of the toner is covered with an external additive. When printing in low image coverage which is printing at a low ratio of occupying image in printing area is increased in the number of times, the consumption amount of the toner in the developer container is reduced, and the supply efficiency of new toner is reduced, and the retention time of the toner in the developer container is prolonged. Therefore, the toner suffers stress due to stirring for a long period of time and the charging amount of the toner is reduced due to embedding of the external additive on the toner surface. Due to the reduction in the charging amount of the toner, a phenomenon such as reduction in the image quality, toner splash, or gray background appears remarkably, causing problems.
As a result of detailed examination on the reduction phenomenon of the charging amount of toner by the inventors of the present invention, it is found that the phenomenon is apt to vary with the particle diameter of the toner. It will be explained below in detail by referring to FIGS. 3 and 4.
FIGS. 3 and 4 show the charging amount distribution conditions of a developer measured by E-spart Analyzer manufactured by Hosokawa Micron, Ltd. In FIGS. 3 and 4, 2000 toner particles to be measured are divided into groups of 400 toner particles (20% of the whole) with small-particle diameters less than 4.45 μm, 1200 standard toner particles (60% of the whole) with particle diameters between 4.45 and 5.68 μm, and 400 toner particles (20% of the whole) with large-particle diameters more than 5.68 μm, and the curves indicated by Entire diameter, Small diameter, Standard diameter, and Large diameter show Q/D distributions of the whole and respective groups.
FIG. 3 shows the distribution conditions of a charging amount of Q/D (hereinafter, may be referred to as simply Q/D) per unit particle diameter of toner in the developer at start time when an image is formed by the image forming apparatus shown in FIG. 1. FIG. 4 shows the distribution conditions of Q/D after an image at a ratio of occupying image in printing area of 0.05% in low image coverage is printed 5000 times in the condition at start time shown in FIG. 3.
As clearly shown in FIGS. 3 and 4, the distribution condition of Q/D varies with the particle diameter of toner, and the reduction in Q/D of the toner on the small particle diameter side is larger, and with respect to the developer used here, in the toner on the small particle diameter side, deteriorated toner causing toner splash and image quality deterioration is apt to be generated.
Therefore, the particle diameter of toner to be developed is selective, and toner with a larger particle diameter has a better developing performance, and furthermore it depends on the size of the developing electric field. When toner with a small particle diameter whose charging amount is reduced due to the selectiveness is increased in quantity, an increase in toner splash or gray background occurs.
In Japanese Un-examined Patent Publication No. 5-224520, it is described that, in the two-color image forming method, to discharge toner charged at reverse polarity or another-colored toner mixed from the developing device, the number of revolutions of a sleeve (may be referred to as a developer carrying member) is set to a number of revolutions larger than that at time of development. However, in this case, toner splash and carrier adhesion are increased. Further, the Japanese Un-examined Patent Publication No. 5-224520 describes that the DC bias voltage at time of toner discharge is lowered, thus the developing electric field to the reverse polarity toner is made larger, and unnecessary reverse-polarity toner is much adhered to the non-image part and is discharged. The developing electric field is made larger, thus unnecessary toner is discharged. However, lowering the DC bias voltage results in making the developing electric field smaller, so that it is not appropriate to discharge toner at a low charging amount.
In Japanese Un-examined Patent Publication No. 2000-293023, a problem is described that when a blank pulse is used as a developing bias voltage, among the toner in the developer, toner with larger particle diameters is consumed much, and toner with smaller particle diameters remains in the developing unit, so that when the development is repeated, the toner particle size distribution is biased toward the smaller-particle diameter side, and the image quality is lowered such that the image density is lowered. Furthermore, it is also described that to discharge the toner with smaller-particle diameters remaining in the developing unit, at time of forced discharge, development using a rectangular pulse is repeated and toner with smaller-particle diameters is consumed much. However, the charging amount of toner is not taken into account and technical thought of discharging deteriorated toner whose charging amount is reduced is not disclosed.
In Japanese Un-examined Patent Publication No. 2004-125829, it is described that in a one-component developing device for applying a DC voltage as a developing bias voltage and developing an image, when the number of image dots at a predetermined number of revolutions of a developer carrying member is a predetermined value or smaller, the DC developing bias voltage is lowered and is set to a voltage equal to the exposure potential, and at time of other than image formation, the developer is developed from the developer carrying member onto the image carrying member, thus deteriorated toner is discharged. However, lowering the DC bias voltage results in making the developing electric field smaller, so that it is not appropriate to discharge toner at a low charging amount.
In Japanese Un-examined Patent Publication No. 11-316490, it is described that in the cleaning mode, an AC bias voltage with a small amplitude is used as a developing bias voltage having a lower developing capacity than that in the ordinary image forming mode, and the developing bias voltage is applied to develop a predetermined image, and the predetermined image is transferred to a transfer material and is discharged ont of the fixing device, thus toner at a small charging amount (weakly charged toner) can be separated from the developer in the developing unit, and a defective image due to inferior toner generated with time is prevented, and the recycling property of toner can be enhanced. However, smaller-particle diameter toner has strong adhesion force with the carrier, so that when the amplitude of the AC bias voltage is made smaller, the force for separating the toner from the carrier is weakened, thus it is difficult to discharge smaller-particle diameter toner having a small charging amount.
Patent Document 1: Japanese Un-examined Patent Publication No. 5-224520,
Patent Document 2: Japanese Un-examined Patent Publication No. 2000-293023,
Patent Document 3: Japanese Un-examined Patent Publication No. 2004-125829,
Patent Document 4: Japanese Un-examined Patent Publication No. 11-316490.
When forming an image in low image coverage, the consumption amount of toner is reduced, and the toner retention time in the developer container is prolonged, thus the time for the toner to suffer stress is prolonged, so that the charging amount of toner, particularly toner with smaller-particle diameters is lowered remarkably, and a problem of toner splash or gray background arises.